A generic lithographic process is presented that allows the fabrication of high density organic electrochemical transistor arrays meant to interface with aqueous electrolytes. The channels of the transistors, which were 6 µm long, were made of the conducting polymer poly (3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) and were in direct contact with phosphate buffered saline. Source and drain electrodes and interconnects were insulated by parylene C, a biocompatible material. The transistors operated at low voltages and showed a response time of the order of 100 µs.